Fix outdated Lets Encrypt client reference. (#2099)
Fix outdated Lets Encrypt client reference. (#2099)

As a follow-up to #2098 this PR changes the sentence describing "an alias for letsencrypt" to reference an "alias to certbot" instead. This fixes what was an outdated reference to the client formerly known as Let's Encrypt, now Certbot.
bmw/boulder,jcjones/boulder,bmw/boulder,ZCloud-Firstserver/boulder,ibukanov/boulder,jcjones/boulder,jfrazelle/boulder,ibukanov/boulder,jcjones/boulder,jfrazelle/boulder,lucasallan/boulder,ZCloud-Firstserver/boulder,benileo/boulder,sophie-h/boulder,ibukanov/boulder,letsencrypt/boulder,sophie-h/boulder,lucasallan/boulder,jfrazelle/boulder,bmw/boulder,ZCloud-Firstserver/boulder,benileo/boulder,lucasallan/boulder,ZCloud-Firstserver/boulder,benileo/boulder,letsencrypt/boulder,jsha/boulder,benileo/boulder,jsha/boulder,jsha/boulder,jcjones/boulder,jfrazelle/boulder,sophie-h/boulder,letsencrypt/boulder